The photomorphogenic control of hypocotyl extension growth was characterized in wild type (WT) and long hypocotyl (Ih) mutant seedlings of cucumber (Cucumis sativus L.) grown under natural radiation in outdoor and glasshouse experiments. Hypocotyl extension growth of WT plants was promoted by supplementing sunlight with far-red light during the photoperiod, by reducing the amount of blue light reaching either the whole shoot or the hypocotyl, and by reducing the amount of UV reaching the whole shoot.The Ih seedlings only responded to a reduction in UV-B levels. Both WT and Ih seedlings showed phototropic responses to the direction of blue light. Increasing degrees of vegetational shade promoted hypocotyl growth of WT plants. The Ih mutant showed no hypocotyl growth promotion by natural shade in glasshouse experiments (no UV-B, low water demand) and a reduced response (10-23% of the WT response, according to pretreatment conditions) in outdoor experiments (UV-B, high water demand).